1. Field of the Invention
The present invention relates to a dedicated short range communications (hereinafter also referred to as DSRC) system that provides a warning of the approach of another vehicle in vehicle to roadside communications, and more specifically, it relates to a mobile station and a base station used in such a dedicated short range communications system.
2. Description of the Related Art
There have hitherto been known conventional dedicated short range communications systems which are used in automatic or electronic toll collection for toll roads or the like so as to execute electronic toll collection applications through data communications. Communication frames in the form of radio or wireless transmission signals used at that time are described, for example, in “Dedicated Short Range Communications (DSRC) System Standard ARIB STD-T75” provided by Association of Radio Industries and Businesses, a public service corporation in Japan. In addition, as a result of toll collection, instruction information is sent from a base station to a mobile station through data transmission so as to notify the mobile station of the permission or non-permission of passage, communication abnormality, or the like (see, for instance, a first patent document: Japanese patent application laid-open No. 2000-307506).
Moreover, dedicated short range communications systems are also used in software applications in which a base station detects the travel speed of a vehicle having a mobile station installed thereon through data communicated between the base station and the mobile station, so that danger, if any, is notified from the base station to the mobile station based on location information on the base station (see, for instance, a second patent document: Japanese patent application laid-open No. 2001-109993).
In conventional dedicated short range communications systems, a dedicated short range communications mobile station (hereinafter referred to as a DSRC mobile station) is not provided with any means for detecting the existence of other mobile stations, and hence in software applications such as electronic toll collection system applications, parking lot applications, etc., for instance, a crossing gate might not sometimes be opened to interrupt traffic of a vehicle such as a car depending upon a variety of conditions such as communication errors, non-insertion of a card into an associated mobile station, etc. In this case, the operator of the car with a mobile station mounted thereon is forced to rapidly stop the car in order to avoid colliding with the crossing gate, but when the operator of the following car is late in noticing the stoppage of the preceding car, the following car cannot occasionally avoid collision against the preceding car.